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Bridge System Safety and Redundancy

A good quantification of redundancy is not currently available to bridge engineers. Construction costs may be needlessly high on some bridges due to member redundancy, and less redundant existing structures may go unidentified. Redundancy, operational importance, and ductility can be considered during design by using load modifiers from the <em>AASHTO LRFD Bridge Design Specifications</em> ranging from 0.95 to 1.05 each. However, such factors should be better specified according to system type, and should be on the resistance side of the equation. A framework for evaluating redundancy in highway bridges is described for superstructures in <em>NCHRP Report 406</em>. For superstructures, a "system factor" between 0.8 and 1.2 is applied based on the girder spacing and number of girders in the system. However, the charts are limited to steel and pretensioned I-beam-slab bridges, and only a small general bonus is given for diaphragms. An alternative methodology is provided to generate the system factor using nonlinear analysis and incrementally increasing the HS20 load, but ability to transmit load longitudinally is still not addressed. Similarly, substructure redundancy is addressed in <em>NCHRP Report 458</em>. System factors are provided for confined and unconfined 2- and 4-column piers; spread footings, drilled shafts, and piles in various soil types. A direct redundancy analysis procedure is also provided. Any ability of the superstructure to enhance the substructure redundancy is ignored. The quantification of redundancy in highway bridge structures is necessary before reliability-based non-collapse criteria can be developed for blast loads, ship-impact, storm surges, or seismic-force effects. A fresh approach is needed to merge past efforts that developed super- and substructure redundancy independently, to quantify the transfer of lateral loads in the superstructures inclusive of multi-cell cross-sections, and to consider the affect of superstructures on substructure redundancy especially in the case of framed structures and super-substructure connections made fixed for live load. The objective of the research is to combine the techniques presented in <em>NCHRP Reports 406 </em>and<em> 458</em>; expand the work to include lateral loads on superstructures, framed systems, single- and multi-cell boxes; and demonstrate the suitability of the proposal within the framework of the previously defined ultimate (strength), functionality (service), and damaged ultimate (collapse) limit states. The following tasks are envisioned: (1) Review <em>NCHRP Reports 406 </em>and<em> 458</em>, as well as Interims to the LRFR Manual for System Factors in Segmental Bridges. Do a literature search to see if any other related studies have since been done. (2) Expand on how the quantity and quality of intermediate and end diaphragms in the superstructure play a role in redundancy. (3) Expand to include single- and multi-cell cross-sections where member resistance may or may not be relevant since a "whole-width" design is often done. (4) Make any necessary updates to the functionality limit state for the recent displacement-based "LRFD Guidelines for Seismic Design of Bridges." (5) Develop a methodology for accessing the redundancy in both the super- and substructure when the superstructure contributes to the substructure's lateral load resistance. (6) Perform parameter studies to show that results are reasonable for all structure types, regardless of girder spacing, number of girder webs, number of substructure units, etc. (7) Develop a list of design examples to illustrate the proposed methodology for three limit states and structure types. List to be approved by project panel. (8) Prepare draft specification changes for both design and rating. (9) Present findings to the AASHTO SCOBS T5 Loads and Load Distribution Committee. The ability to quantify "additional" capacity in bridges due to system redundancy is crucial in understanding bridge performance when subjected to malicious attack, vessel collision, earthquake, or storm surges. Bridge Owners need this information to help determine which existing bridges are most vulnerable due to lack of redundancy, and how to provide adequate redundancy in new bridges. <div></div>